Mobile communications devices have become an integral part of society over the last two decades. The typical mobile communications device includes an antenna, and a transceiver coupled to the antenna. The transceiver and the antenna cooperate to transmit and receive communications signals.
Before transmission, the typical mobile communications device modulates analog voice or digital data onto a radio frequency (RF) signal. As will be readily appreciated by the skilled person, there is a plurality of modulations available for most applications. Some particularly advantageous modulations include, for example, continuous phase modulation (CPM). The constant envelope characteristics of this modulation provide for lower energy demands on the power amplifier of mobile communications devices, for example, by reducing the peak-to-average power ratio (PAPR), increasing average transmit power (providing greater transmission range), and increasing amplifier efficiency, i.e. allowing the use of non-linear amplifiers such as Class C amplifiers. Moreover, CPM provides for efficient use of available bandwidth.
On the other end of the transmission, the receiver device receives the modulated signal. The receiver demodulates the modulated signal, which is then subject to further baseband level processing. Typically, the carrier frequency of the modulated signal is much greater than the bandwidth of the corresponding baseband signals. Accordingly, when the modulated signal is received, the receiver device downconverts the signal frequency to the baseband frequency range.
One approach to communications comprises using a superheterodyne receiver. This approach uses frequency mixing to convert the received signal to a fixed intermediate frequency. The signal is then more readily processed at the fixed intermediate frequency. A potential drawback to this approach is that a superheterodyne receiver may comprise several band pass filters and local frequency sources that increase size, weight, power consumption, and cost.
Another approach to communications is a direct conversion (homodyne) receiver. The direct conversion receiver demodulates the received signal using synchronous detection. The synchronous detection is based upon a local oscillator operating at a frequency close to or identical to the carrier frequency of the received signal. The direct conversion receiver is also known as a zero-intermediate frequency device, since the intermediate frequency conversion stage is omitted.
Although the direct conversion receiver has fewer components than its superheterodyne counterpart, the direct conversion receiver may suffer from other drawbacks. For example, the handling of dynamic range in direct conversion receivers can be complex. Also, direct conversion receivers may suffer from less desirable performance due to direct current (DC) offset of hardware components induced by large RF blockers. This potential drawback can be worsened when the baseband signal out of the receiver has usable spectral components near DC.
One approach to these drawbacks comprises shifting the waveform spectrum away from DC and then applying a high pass filter to the baseband signals to remove the DC component. Nonetheless, this approach has its own drawbacks. For example, unlike direct conversion receivers that have outputs centered at DC, adjacent channel signals create frequency images of the interference that may lie close to, or even overlay the desired signal spectrum. It may be difficult if not impossible to reject the interference image post-receiver by analog or digital filtering. Reduction of this interference image to acceptable levels to meet public safety industry standard interference rejection standards may require precise matching of the phase and gain of the in-phase (I) and quadrature (Q) baseband signals. However, the requisite degree of matching that is required cannot be achieved and/or maintained with commercially available direct conversion receiver circuitry, even if hand-selected to maximize the degree of matching.
One approach to IQ balancing is disclosed in U.S. Pat. No. 8,340,225 to Khoshgard et al. This approach comprises a gain stage, and a phase stage. The I and Q signals are amplitude balanced in the first gain stage, and then output into the second phase stage. The second phase stage phase balances the amplitude balanced I and Q signals.